• Dr. Nikolaos E. Zafeiropoulos
• Dr. Konrad Schneider

Over the years research on the mechanical properties of polymer materials has demonstrated the pivotal importance of understanding the structure and morphology of these materials at the microlevel. The development of synchrotron x-ray scattering techniques has opened up new possibilities for better understanding the properties'-structure relations in polymer and polymer-based materials. In particular, much of our research takes place at the ID13 microfocus beamline of the European Synchrotron Radiation Facility (ESRF) located in Grenoble, France.

The main aims of our research effort are:

1. application of microfocus x-ray scattering as a microscopy tool applied on polymers
2. development of instrumentation for mechanical measurements in situ with x-ray scattering
3. understanding of the mechanisms operating during the deformation of polymers and polymer-based materials (composites, nanomaterials, blends, etc.)

Typical examples are shown here.

The main focus for research is the elucidation of deformation mechanisms and early stages of fracture in polymers. A typical example is the study of damaged area in PA6. For this reason microfocus SAXS and WAXS experiments were conducted on a pre-damaged specimen of PA6, with a scan step of 100 mm, and a beam spot of 5 mm. Figure 1 shows the composite image of SAXS patterns from the scanned zone. Multiple interesting features are revealed with the capability of high spatial resolution of the microfocus beam. An extended analysis through modelling of the deformed area patterns (figure 1e) revealed that nanovoids of a platelet (flat 2D particle) shape are formed radially around conceptual centres, as well as in the direction of the maximum force. The thickness (as determined from the modelling) of the nanovoids varies with respect to the orientation of the nanovoids and can be seen in figure 2 as a polar plot around the conceptual centre. In figure 3 one can see the representation in real space of the position of nanovoids. In addition, to the deformed area ahead of the crack tip, the area behind the crack tip is filled with material that has undergone severe deformation and the crystalline lamellae have obtained specific orientations in space. This is reflected by the evolution of 2 point and 4 point patterns in the SAXS patterns, representing oriented lamellae, and oriented lamellae with multiple fragmentation of crystalline blocks inside them, as depicted in figure 3.

fig. 1 Composite image from the SAXS patterns of the damaged area

fig. 2 Polar plot of the thickness of cavities with respect to angular cuts from
the 2D SAXS patterns and position inside the deformed material area

fig. 3 A real space representation of the different mechanisms operating during the deformation of PA6:
a) oriented lamellae with biaxially oriented fragmented blocks,
b) oriented lamellae,
c) nanovoids originating from fibrillation parallel to the maximum stress field,
d) nanovoids in the amorphous phase within crystalline lamellae with maximum thickness vertically to the maximum stress field

In a different mode we have extended the application of the method in polyolefins. In this case both in situ and off-line scans have been performed on iPP and HDPE samples. Figure 4 shows the evolution of scattering intensity from SAXS experiments in an iPP sample. One can see the disappearance of the long range order originating from the crystalline lamellae of iPP due to extensive fibrillation, and the increase in intensity due to the formation of nanovoids. It is noteworthy that the position of the long range order does not change over time, it is just simply destroyed due to the destruction of the spherulitic structure and the formation of nanovoids.

fig. 4 The evolution of SAXS patterns with respect to the applicable stress over time

One may also see the evolution of anisotropicity in the SAXS patterns due to the formation and orientation of nanovoids with respect to the applied force. This may be depicted in figure 5, as a 2D grey scale plot, overimposed on the force vs. time plot.

fig. 5 The evolution of orientation in the SAXS patterns with respect to the applicable stress over time

Finally one can probe the deformed area after the on line test, and this may be seen in figure 6.

fig. 6 Composite image from the SAXS patterns of the damaged area in iPP
with post mortem scans (each pattern is obtained with 50 mm step)

One can see in figure 6 the evolution of anistropicity and orientation effects in the SAXS pattern as a function of the position inside the deformed area. This can also be visualised in figure 7.

fig. 7 Total integrated intensity and the variation of orientation superimposed, as calculated from figure 6

In addition to the research effort on identifying the fracture mechanisms in polymers we are also interested in understanding the deformation behaviour of polymers. To achieve this we also employ synchrotron radiation x-ray scattering couple with mechanical testing. Miniaturised samples are routinely used and a typical setup of the instrumentation is shown in figure 8.

fig. 8 A schematic drawing of the experimental arrangement to study deformation phenomena in polymers

In addition to mechanical testing and x-ray scattering the setup also includes a microscope fitted on a CCD camera to observe changes optically during the application of load. The method has been applied so far to a series of polymers such as polypropylene, polyethylene and polyamides, as well as on nanocomposites. It is also possible to record simultaneous SAXS/WAXS patterns by carefully adjusting the experimental setup. In figure 9 the evolution of WAXS patterns with increasing strain for two polypropylene samples (one homopolymer and a copolymer with 5% mol ethylene) is shown.

fig. 9 Comparison of the evolution in WAXS with increasing tensile strain
for (top row) homopolymer and (bottom row) copolymer

As the strain increases the isotropic patterns become gradually more anisotropic. Two distinct spots can be seen appearing on the detector perpendicular to the deformation axis. With further deformation, these reflections become more intense and radially diffuse, whilst 4 additional reflections begin to appear further out on the detector. The resulting 6-point pattern can be assigned to the PP mesomorphic phase, or conformationally disordered PP. Through analysis of the SAXS patterns of the patterns shown in figure 9, i.e. the area very close to the beamstop at the centre of the pattern, interesting information can be obtained for the deformation of the samples. Figure 10 shows the SAXS patterns from the zoomed areas of the patterns in figure 9.

fig. 10 Comparison of the evolution in SAXS with increasing tensile strain
for (top row) homopolymer and (bottom row) copolymer PP

As is evident from figure 10 the homopolymer sample exhibit significantly stronger scattering in the small angle regime as compared with the copolymer. This is even more obvious if one plots the total integrated intensity as a function of applied strain, shown in figure 11.

fig. 11 Variation in total SAXS intensity with increasing tensile strain for homo-
and co-polymer samples thermally treated at different temperatures

It is interesting to note that for differently thermal treated samples the homopolymer samples exhibited significantly higher intensity in their corresponding SAXS patterns, denoting that there is a much higher number of inhomogeneities developing with respect to increasing strain. These inhomogeneities are nanocavities formed due to the mismatch in the elastic constants between the crystalline and amorphous phases. The introduction of only 5% mol ethylene randomly in the polypropylene chain significantly depresses the formation of nanovoids, which can act as fracture precursors, thus increasing the fracture toughness of the material.

• Dr. Richard Davies, Dr. Christian Riekel,
  ESRF, Grenoble
• Prof. Gad Marom,
  Hebrew University of Jerusalem, Israel
• Prof. Andrzej Galeski,
  Technical University of Lodz/ Polish Academy of Sciences
• Prof. Alexander Korsunsky,
  University of Oxford